1. Field of the Invention
This invention relates to a method and apparatus for measuring frequency or time and more specifically to measuring the frequency of electronic signals or the time period between the occurrence of electronic signals by comparison to a reference frequency or time period.
2. Description of the Prior Art
During the manufacturing or maintenance of an electronic device it is often necessary to adjust the time periods or frequencies of various electronic components contained within the electronic device. For example, during the manufacture of a computer, it may be necessary to adjust the time period of retriggerable monostable multivibrators or voltage controlled oscillators. In adjusting such devices, it is often desirable to be able to time a one microsecond pulse to be plus or minus ten nanoseconds.
There are several known methods which can be used to time the period of the pulse or to determine the frequency of the oscillator. One method employs a crystal oscillator and consists of counting the number of oscillations made by the output of the crystal oscillator during the period to be timed. The disadvantage of this method of counting the number of pulses or the number of oscillations from a crystal oscillator is that a very fast crystal oscillator must be used with a correspondingly fast counter if a fine resolution is desired. For example, in order to achieve a resolution of plus or minus five nanoseconds, a 200 megahertz crystal oscillator would be required.
The second method involves use of a retriggerable monostable multivibrator, also known as a one shot, as a reference and to compare its time period against the time period or frequency of the electronic component that is being measured. The disadvantage of this technique is that the pulse period of the reference one shot may drift over time. It is, therefore, not a reliable long term standard. The drift of the calibration one shot can result in electronic devices being shipped to customers with improper internal time period adjustments. This slow drift of the one shot calibration standard can, therefore, result in serious problems in maintenance of equipment. For example, if a one shot is originally adjusted to have a 1000 nanosecond time period and is used as a standard to adjust other one shots to a 1000 nanosecond time period, plus or minus 30 nanoseconds, but the one shot calibration standard drifts over time and produces a period of 1020 nanoseconds, the components being measured are adjusted to be within plus or minus 30 nanoseconds of that incorrect standard, which can result in devices being shipped which have one shots with time periods adjusted to be as much as 1050 nanoseconds. These out of tolerance components may cause failures of overall systems. Such failures may go unnoticed initially when the system is installed and become apparent only during usage on a hot day when the time period may further drift to become, for example, 1052 nanoseconds.
Another problem that is often encountered with time or frequency measurement instruments is that, besides being expensive, the instrument itself is necessarily relatively large and it may, therefore, be very difficult to position the instrument in close proximity to the component generating the time period or frequency which is to be measured. This is especially true in the case of automatic test equipment where the frequency or time measurement instrument is but a subordinate, or even optional, part of a larger system. Having the time or frequency measurement instrument in close proximity to the source of the frequency or time period that is to be measured has the advantage that the measurement can be made more accurately. A further disadvantage of some time or frequency measurement instruments is that many of these require that the time period being measured be repeated such that if it is desired to measure the period of a one shot, the one shot must be triggered many times and the time period is determined by an averaging method.
Therefore, what is desirable is to have an instrument that can be used to measure frequency or time periods that is relatively low in cost, that is relatively small in size and that has a relatively fine resolution and whose accuracy will not significantly drift over time.